The invention relates to apparatus for detection of small amounts of vapors or gases in an atmosphere of air or other gas. It combines the features of a first drift tube having electric conductive grids with constant potentials and a second drift tube having conductive grids at modulated potentials. By thus combining the two drift tubes, advantages of both are attained resulting in performance which is superior to that of either one of them individually.
Prior art apparatus for detection of trace gases includes ionization cells such as described in U.S. Pat. No. 3,835,328. In the ionization cell, trace gases are detected by directing a sample of gas past a source of ionizing radiation and then through a recombination region to an ion collector.
An alternative means for detection of trace gases or vapors is a drift tube. In a drift tube, vapors or gases are subjected to ionizing radiation in the same manner as in the ionization cell. In a drift tube, however, the resulting ions are placed in an electric field, causing the ions to migrate in a predetermined direction, where different types of ions can be separated, detected, and measured by virtue of the difference of velocity or mobility of the ions in an electric field. Ion shutter grids or gates are provided for segregating the ions in accordance with their drift time.
U.S. Pat. No. 4,119,851 teaches an apparatus which achieves improvement in selectivity of vapors or gases by combining the features of an ionization cell and a drift tube. The apparatus can take form in two basic configurations. In one configuration, the ionization cell operates as a pre-selector or pre-filter which eliminates or reduces the effects of the great majority of possible interfering ion species while allowing a significant fraction of the ions of interest to pass through. The drift tube then receives the selected ions and further classifies the ions on the basis of their mobility. In an alternate configuration, the drift tube is positioned upstream of the ionization cell.